I. Field of the Invention
The present invention relates generally to group communication systems and more particularly to a method and apparatus for providing fair access to a push-to-talk communication system for all members operating within the system.
II. Description of the Related Art
Point-to-multipoint communication systems have been used for many years to provide communications generally between a central location and multiple users of the system. For example, dispatch systems using Land Mobile Radios (LMRs) have been used in trucks, taxis, buses, and other vehicles in order to communicate scheduling information between a central dispatch center and one or more corresponding fleet vehicles. Communications may be directed at a specific vehicle in the fleet or to all vehicles simultaneously.
Another example of a point-to-multipoint communication system is a group or push-to-talk system. Such a system allows a group of individuals, each having a wireless communication device, to communicate with other members of the group. Typically, a group system relies on a single frequency, or dedicated channel, over which communications are received by the wireless communication devices. Only one member may transmit information to the other members at a time. However, all members can listen to the dedicated broadcast channel to receive communications from the single member who is transmitting. Members desiring to communicate with other members of the system typically send an access request by depressing a push-to-talk button on their respective communication devices which allows them sole access to the dedicated channel. Hence, these systems are known as push-to-talk communication systems.
Push-to-talk systems are typically used in outdoor settings where a group of people, or members, require communications with each other in a "point-to-multipoint" fashion. Examples of push-to-talk system uses include workgroup communications, security communications, construction site communication, and localized military communications. The group of people requiring communications with each other is commonly known as a "net," each member of the net sometimes referred to as a "net member."
In a typical push-to-talk system, a dedicated channel, sometimes referred to as a broadcast channel, is used to transmit communications from one member to multiple other members of the net simultaneously. The dedicated channel may comprise a single channel or frequency, or a group of individual channels managed by a controller to imitate the single channel. In either case, only one member may transmit voice and/or data communications to the other member users at any given time. If another member attempts to transmit voice and/or data over the broadcast channel while another member is transmitting, interference between the two competing communications will occur, resulting in non-intelligible voice and/or data being received by the other net members.
To prevent interference between members, many push-to-talk systems use an arbitration system to limit access to the broadcast channel to only one net member at any given time. Typically, the arbitration system is located at a central facility, base station, or other communication facility where communications between net members pass. The arbitration system typically comprises a digital computer which monitors transmissions from members for an access request signal. The access request signal is generated upon a net member depressing a push-to-talk button, located on the wireless communication device. The access signal is transmitted from the wireless communication device to the arbitration system prior to a net member commencing transmissions.
Generally, the arbitration system will allow a requesting member to transmit information over the broadcast channel if no other member is presently using the broadcast channel. If the broadcast channel is in use when a member requests access, the arbitration system may deny access to the requesting member. One problem with such an arbitration arrangement is that the probability of successfully acquiring the broadcast channel remains constant, no matter how many times a particular member has been denied. Thus, a member wishing to communicate important information to other members has no way of increasing his or her chances of obtaining access to the broadcast channel, despite repeated attempts to do so.
Another problem with present arbitration systems is that certain members may never get a chance to transmit on the broadcast channel if a large number of member users are constantly requesting access to the channel. In fact, as the number of members requesting access to the broadcast channel increases, the chances of any one net member being denied access increases proportionately.
Some arbitration systems have tried to overcome these problems by providing each member with a priority code. The priority code is generally preassigned to member users and remains unchanged. When a first member is transmitting over the broadcast channel and a second member having a higher priority code transmits an access request signal to the arbitration system, the arbitration system may discontinue the lower priority transmission of the first member and allow the higher priority transmission of the second member to take place. Such a priority scheme is problematic in that member users having high priority may dominate the communication system, preventing lower priority users access to the broadcast system.
What is needed is an arbitration system that will allow true fair access to a push-to-talk communication system without the problems inherent in present priority schemes.